[Beowulf] Home beowulf - NIC latencies

[Joachim Worringen]

Josip Loncaric wrote:
> Are there any projects that would expand the ability of MPI application 
> programmers to provide performance hints to the MPI library?  For 
> example, hints indicating that certain messages are latency sensitive 
> whereas others need optimal bandwidth and low CPU overhead?

MPI offers a lot of different send modes already. If you use a ready 
send, the MPI library can assume that you are interested in low-latency 
delivery; if you use a non-blocking send, it should be o.k. for the 
library to assume that you are interested in overlapping computation and 
communication and so on. On the receiving side, a hybrid 
polling-blocking approach for receiving can be applied.

I do not think that there is serious demand for more explicit "steering" 
of the MPI library. User's make much to little use of the existing ways 
(that I described above). But, if you really want to do such stuff, you 
could use (implementation-specific) attributes which you assign to 
different communicators, one for "low-latency" delivery and one for 
"low-cpu", or whatever. But this has more effect on the sending side 
than on the receiving side. I wouldn't invest work into this unless you 
have very good reasons. Esp. as this would be non-portable, few users 
would ever take notice.

  Joachim

-- 
Joachim Worringen - NEC C&C research lab St.Augustin
fon +49-2241-9252.20 - fax .99 - http://www.ccrl-nece.de